The present invention relates to a range finding arrangement or distance measuring arrangement and more particularly, to a distance measuring arrangement capable of obtaining distance signal based on outputs from photoresponsive elements for use, for example, in a camera provided with an automatic focus control system.
Commonly, there is a tendency for responsive characteristics of photoresponsive elements to be deteriorated with respect to a low intensity of illuminance. Accordingly, in the conventional distance measuring arrangements employing photoresponsive elements, there are such drawbacks that, against target objects or areas with a low intensity of illuminance, errors are involved in distance signals due to delays in the response of the photoresponsive elements and related circuits, making it impossible to carry out accurate distance measurements, and thus, the target objects or areas to be measured for determining their distance are undesirably limited.
More specifically, one example of the conventional distance measuring arrangements in which distance signals are obtained on the basis of outputs from photoresponsive elements will be described hereinbelow with reference to FIG. 1.
The known distance measuring arrangement A of FIG. 1 which is arranged to obtain signals indicative of distances based on the trigonometric distance measurement by taking correlation of spatial images formed on a pair of photoresponsive elements 1a and 1b, for example, of photodiodes, etc., includes a stationary mirror 2a, a movable mirror 2b for scanning, the pair of photoresponsive elements 1a and 1b disposed in a spaced relation between the mirrors 2a and 2b through corresponding image forming lenses 3a and 3b for forming images of a target object aa' on said photoresponsive elements 1a and 1b, an eccentric cam 4 to be driven for rotation by driving means (not shown), a synchronous switch S1 selectively opened or closed through the rotation of the cam 4, an absolute value inversion circuit 5 which is coupled to the photoresponsive elements 1a and 1b, and a main extreme value detection circuit 6 which is coupled to the absolute value inversion circuit 5 and also to the switch S1. The eccentric cam 4 is arranged to be rotated at a predetermined velocity .omega.t by the driving means (not shown), while a support member 7 supporting, at its one end, the movable mirror 2b is urged by a spring 8 to contact, at its other end, the peripheral edge of the cam 4 under pressure. By the above arrangement, the movable mirror 2b is caused to pivot through the support member 7, at a predetermined cycle as shown in FIG. 2(b), following the rotation of the eccentric cam 4, and the angle of deviation of the mirror 2b is represented by .theta.=.alpha.(1-cos.omega.t)-.theta.', where .theta.' denotes the angle of deviation at the initial position of the movable mirror 2b and .alpha. represents a constant.
On the other hand, the absolute value inversion circuit 5 including, for example, a differential amplifier, an inverter, etc. (not shown) is so arranged as to invert the absolute value of the difference between outputs of the photoresponsive elements 1a and 1b, and develops an output voltage V as shown in FIG. 2(c). Meanwhile, the main extreme value detection circuit 6 includes a resistor 10 which is connected to the non-inverting input terminal of an amplifier 11, and also includes the switch S1 and a capacitor 9 which are connected to the inverting input terminal of the amplifier 11, and which is further coupled to the output side of the amplifier 11 through a diode 12 and also to an inverter 13, as shown in FIG. 3.
In the above arrangement, when the output voltage V of the absolute value inversion circuit 5 reaches the maximum value, the detection circuit 6 detects such a maximum value for converting the output thereof from a Low level to a High level as shown in FIGS. 2(d) and 2(e). More specifically, the main extreme value detection circuit 6 of FIG. 3 functions as a voltage follower after the switch S1 is opened as in FIG. 2(a) at a time period t.sub.0, and following the output voltage of the inversion circuit 5, the capacitor 9 is charged, and when the output voltage of the inversion circuit 5 passes its maximum value, i.e. when it becomes lower than the voltage across the capacitor 9, the output of the amplifier 11 is inverted to alter the output of the inverter 13 from Low level to High level.
In the known distance measuring arrangement A as described above, the distance l up to the target object aa' may be represented by the relation EQU l=X/tan2.theta. (1)
in which X denotes a base line length and can be obtained as a value of .theta., i.e. a function of time. In other words, on the assumption, for example, that the target object aa' is present in a fixed position at a distance l1, when the angle of deviation of the movable mirror 2b has reached a value .theta.1 given by the above equation (1), the images of the target object aa' formed on the pair of photoresponsive elements 1a and 1b are aligned with each other to produce the same output from the photoresponsive elements 1a and 1b, and thus, the output voltage V of the absolute value inversion circuit 5 reaches its maximum value. More specifically, as shown in solid lines in FIG. 2 (c), in a time period t.sub.1 corresponding to the angle of deviation .theta.1, the output voltage V of the absolute value inversion circuit 5 reaches the maximum value. It should be noted here, however, that the above function relates only to the case where the intensity of illuminance of the target object aa' is high, and that, when the intensity illuminance of the target object aa' becomes low, a delay takes place in the output voltage V of the absolute value inversion circuit 5 as shown by dotted lines in FIG. 2c) due to the delay in response of the output from the photoresponsive elements 1a and 1b, with the result that, upon comparison between the case where the intensity of illuminance of the target object aa' is high and the case where it is low, a delay represented by .DELTA.T(E) takes place in the distance signal which is the output of the main extreme value detection circuit 6 as shown in FIG. 2(e).